Li Hao, Kuninori Suzuki et al. (University of Tokyo 東京大学大学院新領域創成科学研究科) report that lipophilic dye octadecyl rhodamine B (R18) is transported to the ER mediated by membrane transfer proteins. During autophagy, ER-resident R18 is transferred to the autophagic membrane via Atg2. After termination, R18 is reversed back to the ER, showing that the direction of bridge-type lipid transfer is modulated by metabolic states.
Bridge-like lipid transfer proteins (LTPs) contain a repeating β-groove domain and long hydrophobic grooves that act as bridges at membrane contact sites (MCSs) to efficiently transfer lipids. Atg2 is one such bridge-like LTP essential for autophagosome formation, during which a newly synthesized isolation membrane (IM) emerges and expands through lipid supply. However, studies on Atg2-mediated lipid transfer are limited to in vitro studies due to the lack of a suitable probe for monitoring phospholipid dynamics in vivo. Here, we characterized the lipophilic dye octadecyl rhodamine B (R18), which internalizes and labels the endoplasmic reticulum (ER) in a manner that requires flippases and oxysterol-binding protein–related proteins. Using R18, we demonstrated phospholipid transfer from the ER to the IM during autophagy in vivo. Upon autophagy termination, our data suggested the reversible phospholipid flow from the IM to the ER in response to environmental changes. Our findings highlight the critical role of bridge-like LTPs in MCS-mediated phospholipid homeostasis.
